Method and system for distributed acoustic sensing

ABSTRACT

Described herein is a method and system of distributed acoustic sensing, such as in an urban or metropolitan area involving a dedicated and established fibre optic communications network including a data centre. In general, the disclosed method and system includes the steps of (a) selecting an optical fibre cable installation having a path extending across a selected geographical area, the optical fibre cable installation including a bundle of optical fibres and forming part of a fibre-optic communications network, (b) determining characteristics associated with the optical fibre and/or the selected optical fibre installation, (c) transmitting outgoing light in the optical fibre, (d) receiving reflected light back scattered along the optical fibre, and (e) based on the reflected light and the determined characteristics, generating an alert signal representative of an acoustic event. The disclosed method and system may be useful in the detection of acoustic events near or within the selected geographical area.

FIELD OF THE INVENTION

The present invention generally relates to a method of distributedacoustic sensing based on one or more optical fibres. More particularly,the present invention relates to a method of distributed acousticsensing based on one or more installed optical fibre cables.

BACKGROUND OF THE INVENTION

Fibre-optic distributed acoustic sensing can detect acoustic events insurrounding regions along an optical fibre. An acoustic event can becaused by incidents such as underground digging near a gas pipe, waterpipe or a power cable, or pedestrian and road traffic activities.Different types of incidents may cause different acoustic signatures inthe acoustic event. Monitoring of acoustic events therefore allows foralerts to be generated for the prevention or identification of theseincidents, or for tracking of road users in the case of pedestrian androad traffic.

The method of deploying a dedicated optical fibre for distributedacoustic sensing may make sense from a design perspective, such that thefibre optic conditions and parameters (e.g. spatial uniformity along theoptical fibre, trench depths, and levels of acoustic attenuation) areknown or well-controlled upon installation. However, the installation ofa dedicated optical fibre for distributed acoustic sensing can beexpensive and disruptive, particularly in and around an urban centre.

Reference to any prior art in the specification is not, and should notbe taken as, an acknowledgment or any form of suggestion that this priorart forms part of the common general knowledge in any jurisdiction orthat this prior art could reasonably be expected to be understood,regarded as relevant and/or combined with other pieces of prior art by aperson skilled in the art.

SUMMARY OF THE INVENTION

According to a first aspect of the present disclosure, there is provideda method of distributed acoustic sensing, the method including:

-   -   selecting an optical fibre cable installation having a path        extending across a selected geographical area, the optical fibre        cable installation including a bundle of optical fibres and        forming part of an established and dedicated fibre-optic        communications network, the bundle of optical fibres including        an unused channel or unlit optical fibre for communication;    -   determining characteristics associated with the optical fibre        and/or the selected optical fibre installation, including        geospatially calibrating the optical fibre for mapping between        one or more positions along the length of the optical fibre and        a corresponding one or more locations in the geographical area;    -   transmitting outgoing light in the optical fibre;    -   receiving reflected light back scattered along the optical        fibre, the reflected light including fluctuations over time; and    -   based on the fluctuations and the determined characteristics,        generating an alert signal representative of an acoustic event.

The fibre-optic communications network may be an urban or metropolitanarea network. Alternatively or additionally, the fibre-opticcommunications network may be an enterprise network. The enterprisenetwork may include one or more data centres. Where the enterprisenetwork includes a plurality of data centres these are interconnected byoptical fibre installations.

The spatially calibrating step may include generating an acousticcalibration signal at or adjacent the one or more positions along theoptical fibre and detecting corresponding fluctuations at the one ormore locations in the geographical area. It may further includedetermining and logging the geospatial locations of the positions interms of geographic coordinates on the earth's surface, detectingcorresponding fluctuations at the one or more locations in thegeographical area in received reflected light backscattered along theoptical fibre, and determining the path length of the optical fibrecorresponding to the one or more geospatial locations.

The spatially calibrating step may further comprise correlating the pathlength of the optical fibre with the geographic co-ordinates of the oneor more locations to generate a look up table correlating optical pathlength with geographic co-ordinates.

The alert generating step may include determining a location of anoccurrence of an incident in the geographical area based on acorresponding fluctuation detected based on the mapping.

The characteristics determining step may include acousticallycalibrating the optical fibre to reduce impact of unwanted acousticinterference. The acoustically calibrating step may include applying aspectral filter to the fluctuations to band-pass or band-reject theunwanted acoustic interference.

The selected geographical area may include multiple zones correspondingto multiple sections of the optical fibre, and the spectral filterapplying step may include applying the spectral filter with band-pass orband-reject characteristics based on one of the multiple zones orcorresponding sections. Alternatively or additionally, the spectralfilter applying step may include applying the spectral filter withband-pass or band-reject characteristics based on the time of a dayand/or the day of a week.

The characteristics determining step may include physically calibratingthe optical fibre. The physically calibrating step may include obtainingany one or more of the following properties of the optical fibre: coreproperties, attenuation properties, and trench properties. The coreproperties may include core diameter and/or numerical aperture. Theattenuation properties may include propagation loss, existing faultsand/or spliced points. The trench properties may include burialconditions and/or cable enclosure conditions.

It will be appreciated that there are significant variations in thematerial surrounding the trench and cable, including rock, gravel,concrete, sand, water, earth, clay, bitumen or a combination of one ofmore of these. The acoustic impedance of these materials variessignificantly, with the result that there will be variations inimpedance between the perturbation or source of interest and the fibreoptic cable.

Seismic calibration of the surrounding media is performed in a similarway to seismic profiling of the type performed in oil and gasprospecting, involving known techniques for characterising acousticimpedance around well casings. In the present case the objective is notto determine the types and states of surrounding materials but ratherthe acoustic and seismic transfer functions that these materials formspatially between the fiber and the perturbations of interest. Suchtransfer functions allow the heterogeneous media to be accounted for andso allow an accurate estimate of the spatial position, kinetics and thesource frequencies present of any given perturbation around the opticalfiber. The three precursory calibration steps that can be performed thusinclude optical calibration of fiber itself, geospatial calibration, andseismic calibration of surrounding media.

The alert signal generating step may include classifying the alertsignal into one or more classes of alerts based on acoustic signaturesof the fluctuations. The selected geographical area may include multiplezones corresponding to multiple sections of the optical fibre, each zoneor corresponding section being associated with generation of one or moreselected classes of alerts. Alternatively or additionally, the selectedgeographical area may include multiple zones corresponding to multiplesections of the optical fibre, each zone or corresponding section beingassociated with non-generation of one or more excluded classes ofalerts.

The one or more selected or excluded classes of alerts corresponding toeach zone or section of the optical fibre may be related to railmonitoring, road monitoring, and perimeter intrusion detection.

The method may further include switching the transmission of theoutgoing light and the reception of the reflected light to anotherunused channel or unlit optical fibre for communication in, the otheroptical fibre being in another bundle of optical fibres in anotherselected optical fibre cable installation having another path extendingacross another selected geographical area. The switching step mayinclude time-multiplexing the transmission of the outgoing light and thereception of the reflected light to multiple optical fibre cableinstallations.

The method may further include the step of bypass-splicing to bypassconnecting infrastructure. The connecting infrastructure may alsoinclude one or more fibre transfer panels (FTPs) or patch panels.Adjusting band-pass or band-reject frequency range of the spectralfilter and determining the resulting noise level based on the adjustedfrequency range may also occur.

The method may further including determining a speed of the acousticevent and, based on the determination, suppressing or enabling thegeneration of the alert signal representative of the acoustic event.

According to a second aspect of the present disclosure, there isprovided a system for distributed acoustic sensing, the systemincluding:

-   -   a distributed sensing unit for:        -   transmitting outgoing light in an optical fibre;        -   receiving reflected light back scattered along the optical            fibre, the reflected light including fluctuations over time;            and        -   based on the fluctuations, generating an alert signal            representative of an acoustic event, and    -   an optical switch for coupling the distributed sensing unit to a        selected one of multiple optical fibre cable installations, each        installation having a path extending across a respective        selected geographical area and including a bundle of optical        fibres and forming part of a fibre-optic communications network,        the bundle of optical fibres including an unused channel or        unlit optical fibre for communication.

The optical switch and the distributed sensing unit may be located in adata centre connecting to an enterprise network.

The multiple fibre cable installations may be connected to or terminatedat the data centre.

Further aspects of the present invention and further embodiments of theaspects described in the preceding paragraphs will become apparent fromthe following description, given by way of example and with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates an example of a system for distributed acousticsensing.

FIG. 1b illustrates an example of a density plot of electrical signalsgenerated by the system of FIG. 1a over time.

FIG. 2 illustrates an example of a disclosed method of distributedacoustic sensing.

FIG. 3a is a schematic diagram of a data centre cluster includingseveral data centre buildings.

FIG. 3b is a schematic diagram of a single data centre building.

FIG. 4 illustrates an aerial map of different trench conditions.

FIG. 5 illustrates an example of the system of FIG. 1 optically coupledto an optical switch and multiple optical fibre cable installations.

DETAILED DESCRIPTION OF EMBODIMENTS

The principle of fibre-optic distributed acoustic sensing relies on theoccurrence of an acoustic event causing a corresponding localisedperturbation of refractive index of an optical fibre. Due to theperturbed refractive index, an optical interrogation signal transmittedalong an optical fibre and then back-scattered in a distributed manner(e.g. via Rayleigh scattering or other similar scattering phenomena)along the length of the fibre will manifest in fluctuations (e.g. inintensity and/or phase) over time in the reflected light. The magnitudeof the fluctuations relates to the severity or proximity of the acousticevent. The timing of the fluctuations along the distributedback-scattering time scale relates to the location of the acousticevent.

In one example, a unit 100 for use distributed acoustic sensing (DAS) isillustrated in FIG. 1a . The DAS unit 100 includes an opticaltime-domain reflectometer (OTDR) 102. The OTDR 102 includes a lightsource 104 to emit an optical interrogation signal 106. Theinterrogation signal 106 to be sent into the optical fibre 105 may be inthe form of a short optical pulse. The OTDR 102 includes a photodetector108 configured to detect the reflected light 110 and produce acorresponding electrical signal 112 with an amplitude proportional tothe reflected optical intensity. The DAS unit 100 also includes aprocessing unit 114, within or separate from the OTDR 102, configured tomeasure the fluctuations in the electrical signal 112 for determiningthe acoustic event based on the measured fluctuations 116 in intensityas compared between two different times (t₁ and t₂). FIG. 1b illustratesan example density plot combining electrical signals 112 generated bythe DAS unit 100 of over time. The horizontal axis (labelled “Channel”)represents position along the fibre, the vertical axis (labelled “Time”)represents time, and the colour-coded amplitude of the plot representsreflected intensity. In FIG. 1b , features such as straight lines withrelatively constant gradients are associated with moving objects (withthe gradients being indicative of speed) that cause the relevantacoustic events detected by the DAS unit 100. If the OTDR isphase-sensitive, phase fluctuations in the reflected light may beadditionally or alternatively measured. FIG. 1b is also offset to removethe attenuation slope in the electrical signal 112 present in FIG. 1a .The acoustic event being determined may be indicative of specificstationary or moving occurrences, such as excavation, drilling, digging,traffic flows, trains passing by and pedestrian flows.

Described herein is a method of distributed acoustic sensing. Anarrangement of the disclosed method 200 is illustrated in FIG. 2. Ingeneral, the disclosed method includes the steps of (a) selecting anoptical fibre cable installation having a path extending across aselected geographical area, the optical fibre cable installationincluding a bundle of optical fibres and forming part of a fibre-opticcommunications network (step 202), (b) determining characteristicsassociated with the optical fibre and/or the selected optical fibre theinstallation (step 204), (c) transmitting outgoing light in the opticalfibre (step 206), (d) receiving reflected light back scattered along theoptical fibre (step 208), and (e) based on the reflected light and thedetermined characteristics, generating an alert signal representative ofan acoustic event (step 210). The disclosed method may be useful in thedetection of acoustic events near or within the selected geographicalarea.

Rather than deploying a dedicated optical fibre for distributed acousticsensing, the disclosed method relies on selecting an existing opticalfibre cable installation forming part of the fibre-optic communicationnetwork. For example, the fibre-optic communication network may be adatacom network (e.g. to or from a data centre or between data centres)or a telecom network (e.g. to or from a local exchange) or an enterprisenetwork (e.g. to or from large enterprises and cloud and data centresuppliers or between enterprises). While selecting an installation froman existing communication network for distributed acoustic sensing avoidthe expenditure in installing a dedicated cable, a number of technicaldifficulties may need to be overcome. As a result, a skilled personwould not be motivated in selecting an existing fibre-opticcommunication network for distributed acoustic sensing. Further, theskilled person would not recognise the technical difficulties in usingan existing fibre-optic communication network for distributed acousticsensing and would not recognise how to address these technicaldifficulties.

Selection of Optical Fibre Cable Installation

In one example, the disclosed method 200 involves, in step 202,selecting an existing optical fibre installation. The selected opticalfibre cable installation has a path extending across a selectedgeographical area. The selected geographical area may be an urban ormetropolitan area. In one arrangement, the optical fibre cableinstallation includes a bundle of optical fibres in which an opticalfibre is unlit for communications purposes. The unlit optical fibre istherefore able to be used for distributed acoustic sensing. In anotherarrangement, the optical fibre cable installation includes a bundle ofoptical fibres in which one or more of the optical fibres include timeor wavelength channels that are unused for communications. For example,in an enterprise network where dense wavelength-division multiplexing(DWDM) is employed, only some but not all of the DWDM channels carrycommunication traffic. The rest of the DWDM channels may be unused. Oneor more of the unused channels are therefore able to be used fordistributed acoustic sensing. Further references to the use of unusedchannels or unlit optical fibres are applicable to either arrangement.

One or more factors may affect the selection of the optical fibreinstallation. For instance, an appropriate selection is based on aninstallation that forms part of an existing enterprise network. Unlike adedicated optical fibre which has an end-to-end geometry, an enterprisenetwork connects to multiple optical fibre cable installations, forexample, via one or more data centres or hubs. A data centre representsthe aggregation of servers and storage, with the advantage of having ahigh volume of fibres and fibre cables. In one arrangement, the multipleoptical fibre cable installations may span different regions of theurban or metropolitan area. Selecting an enterprise network thereforefacilitates a relatively large geographical coverage for distributedacoustic sensing through accessing multiple optical fibre cabletermination or connection points at or near a central location (e.g. adata centre or hub).

In one arrangement, the selection of an optical fibre installation maybe based on selecting an optical path passing through multiple datacentres. For example, in a hub-and-spoke configuration, a central datacentre is connected to multiple nearby data centres. As another example,in a data centre cluster, multiple nearby data centres areinterconnected. FIG. 3a illustrates an example of a data centre cluster300 including four data centre buildings 302 a, 302 b, 302 c and 302 din communication with one another. The data centre buildings 302 withinthe cluster 300 share a transit communication fibre cable 304 thatallows communication externally from the cluster 300 to remote serviceproviders (such as carriers and content providers). Within the cluster300, the data centre buildings 302 are communicatively interconnected byintra-data-centre communication fibre cables 306 in trenches thatcontain cross connects between server banks in different data centrebuildings 302. Within each data centre building 302 are cross-connects308. The intra-data-centre communication fibre cables 306 representscritical infrastructure to data centre operators as the serviceproviders are not their responsibility and a break in theintra-data-centre communication fibre cables 306 will affect customerhosting, where there is no SDN overlay protections as with carrier meshnetworks. Each of the transit communication fibre cable 304,intra-data-centre communication fibre cables 306 and cross-connects 308represent a fibre-optic asset. The selection of an optical fibreinstallation may be based on selecting an optical path that passesthrough most if not all fibre-optic assets to be protected.

Within each data centre, such as that shown at 302 e in FIG. 3b , fiberbreakout cabinets 310 a and 310 b are shown for receiving respectiveoutdoor fiber optic cables 312 a and 312 b in fiber transfer panels 314a and 314 b. The disclosed method 200 may include a step ofbypass-splicing. Bypass-splicing refers to a thermal splicing step inwhich the sensing fiber 318 is spliced at 320 so as to bypass connectinginfrastructure such as the one or more fibre transfer panels (FTPs) 314a and 314 b or patch panels. Some of the fiber optic cables 322 extendfrom the breakout cabinets 310 a and 310 b to building customer cabinets324. Others are optical cross connect cables as is shown at 326.

Deploying a coherent OTDR requires a tighter tolerance to backreflections than a standard telecommunications transceiver (e.g. asfound in an enterprise network). The use of flat connector type, such asFC-PC which are commonly found on FTPs, create unwanted levels of backreflections. The step of bypass-splicing provides that, in a data centreenvironment, the sensing fibre 105 in the selected cable installationused for distributed acoustic sensing (or the sensing fibre 118) is notconnected to the FTP(s) or patch panel(s). In one arrangement,bypass-splicing makes the sensing fibre 105 or 118 continuous from thepatch lead at the DAS unit 110 to a termination unit. The disclosedmethod 200 may also include a step of determining whether the sensingfibre 105 or 118 passes through any connecting infrastructure. Inaccordance with the determination, the disclosed method 200 may includeremoving parts of the sensing fibre 105 or 118 corresponding to theconnecting infrastructure, and splicing in a bypass fibre asreplacement.

Further, the selection of an enterprise network over a long-haul networkrelates to an observation that a relatively short reach network requiresno regeneration or amplification, and therefore tends to have arelatively large number of optical fibres in the bundle (i.e. arelatively large cross-section), in which there is an increasedlikelihood of one or more unused channels or unlit optical fibres. Theunused channels or unlit optical fibre(s) may have been laid as part ofa fibre-optic bundle to act as spare capacity allowing future growth innetwork demand. In some embodiments if all fibres are deployed then itis also feasible that a deployed but say less critical fibre may berepurposed. In contrast, a long-haul network may not be an appropriateselection since the long reach of the network requires regeneration oramplification. A long-haul network therefore tends to have a relativelysmall cross-section, in which there is a decreased likelihood of anunused channel or unlit fibre, or a fibre that is not performing acritical role.

An example of a large cross-section optical fibre cable is Prysmian's (aregistered trade mark of Prysmian Cavi E Sistemi Engergia s.r.l.) MultiLoose Tube Duct Cable having 216 to 624 fibres. In other examples, alarge cross-section may mean an optical fibre cable having 32 to 64optical fibres. In some configurations, the reach of an urban ormetropolitan area network is less than about 50 to 100km in reach. Therelatively short reach is limited by attenuation and optical receiversensitivity where regeneration or amplification is not used. In otherconfigurations, the present disclosure is not limited to an urban ormetropolitan area network nor is it reach-limited to 50 to 100 km. Thepresent disclosure is applicable to other communication networks havingan unused channel or unlit optical fibre for communication purposes(e.g. international submarine optical fibre cables).

Alternatively or additionally, the selection of optical fibre cableinstallation may be based on proximity to existing infrastructure. Suchexisting infrastructure may include but is not limited to: roads, rail,water, power, electricity, telecommunications, data centres, buildings,bridges, tunnels, pedestrian access ways, rivers, harbours, lakes,docks, construction sites, industrial parks, and criticalinfrastructure. Still alternatively or additionally, the selection ofoptical fibre cable installation may be based on stakeholder types inaccordance with the audience of the alert signal. The stakeholder typesmay include but are not limited to: emergency and disaster management,critical infrastructure management, citizen services, publicadministration services, law enforcement, and enterprise security &asset management.

Once an optical fibre cable installation is selected, the disclosedmethod 200 involves optically coupling (not shown) the DAS unit 100 tothe selected installation. Depending on the termination types of theoptical fibre 105, the optical coupling may involve splicing (if thetermination type is a bare fibre) and/or connecting (if the terminationtype is an optical connector such as SC or FC connector). Where theacoustic event is determined, an alert signal representative of theacoustic event may be generated. The alert signal generating step mayinclude classifying the alert signal into one or more classes of alerts(e.g. excavation threats, heavy pedestrian traffic, heavy roadwaytraffic, etc) based on acoustic signatures of the fluctuations. Sometechniques in alert classification are summarised and further referencedin, for example, “Fiber Sensing: Optical fiber monitors the arterialnetworks of commerce”, Laser Focus World, volume-51, issue-08, 8 Jun.2015(http://www.laserfocusworld.com/articles/print/volume-51/issue-08/features/fiber-sensing-optical-fiber-monitors-the-arterial-networks-of-commerce.html).In one configuration, the geographical area is divided into multiplezones corresponding to multiple sections of the optical fibre. In thisconfiguration, each zone or corresponding section is associated withgeneration of one or more selected classes of alerts (or non-generationof one or more excluded classes of alerts). For example, each zone maybe represented by a different stakeholder. Where the stakeholder is autility connection operator (e.g. for supplying gas, power or water),the classes of alerts selected for generation may be associated withdrilling, excavation or digging near a supply cable. Additionally, theclasses of alerts excluded for generation may be associated withpedestrian traffic or roadway traffic. On the other hand, where thestakeholder is a transit operator (e.g. bus or rail operator), theclasses of alerts selected for generation may be associated withpedestrian traffic or roadway traffic. Additionally, the classes ofalerts excluded for generation may be associated with drilling,excavation or digging near a supply cable. The one or more selected orexcluded classes of alert along the same optical fibre for differentzones or sections are related to rail monitoring, road monitoring, andperimeter intrusion detection.

Acoustic Calibration

The disclosed method 200 also involves, in step 204, determiningcharacteristics associated with the optical fibre and/or the selectedoptical fibre the installation. In one example, this determining step204 includes acoustic calibration.

An urban or metropolitan area over which the distributed acousticsensing is conducted is likely an area with unwanted acousticinterference. The unwanted acoustic interference may interfere, mask orotherwise affect the characteristics of the acoustic event beingdetermined. In one configuration, to reduce the impact of unwantedacoustic interference, the disclosed method 200 includes applyingspectral filtering to the detected fluctuations to reduce or removefluctuations associated with the unwanted acoustic interference. Forexample, acoustic interference arising from bus engine noise maytypically range from 1 to 120 Hz due to the engine's low revolutionrates at low travelling speeds. The low revolution rates result in lowacoustic frequencies which are more penetrating compared to higheracoustic frequencies. To reduce the impact of such penetrating noise,the detected fluctuations may be spectrally filtered to remove orattenuate low frequency fluctuations by, for instance, a low-pass filterhaving a cut-off frequency around 100-150 Hz.

Further, the disclosed method may selectively apply the spectralfiltering to one or more zones of the geographical area. Different zonesof the geographical area may require different or no spectral filtering.For example, away from an urban zone and on a highway, there may be noneed to apply the low-pass filter but the disclosed method may apply aband-reject filter to remove or attenuate tyre noise. Alternatively oradditionally, the disclosed method may selectively apply the spectralfiltering to fluctuation based on time, e.g. the time of the day or theday of the week.

To determine the appropriate band-pass or band-reject frequencies for aparticular zone or particular time, the step 204 may include obtainingbaseline data of frequency content along the optical fibre. The baselinedata may be separately obtained for individual zones and/or particulartimes. The baseline data may be obtained by monitoring fluctuationsduring a specific duration. The frequency content in the baseline datamay be based on averaging several sets of data measured over thespecific duration. The specific spectral filter to be used based on thezone or time may be configured to have a band-pass or band-rejectprofile which is opposite (e.g. inverted) the frequency content in thebaseline data.

In one arrangement, the disclosed method 200 includes adjusting theband-pass or band-reject frequency range. The disclosed method 200 mayfurther include determining the resulting noise level based on theadjusted frequency range. The adjustment may be dynamically implementeduntil the noise level is below a particular threshold. Alternatively oradditionally, the adjustment may be recursively implemented until thenoise level is below the particular threshold or another threshold. Thenoise level may be determined based on the noise bandwidth at eachcalibration site. The noise bandwidth may be determined by the channelanalysis and acoustic waterfall functionality in the DAS software, andselecting appropriate waterfall and excitation frequencies for eachcalibration site. In one arrangement, the noise bandwidth is determinedby implementing a filter having an integration time which issignificantly longer than the transient signals of interest. Forexample, the noise background may be calculated based ontime-integration over tens of minutes to relate to the threshold for thebackground noise over a range of positions along the fibre length (e.g.corresponding to a telecom pit below ground level). The differencesbetween the acoustic intensity of the background noise compared with theintensity of the electrical signal 112 (e.g. relating to the telecompit) provides an indication of the noise bandwidth.

Spatial Calibration

An optical fibre cable installation does not extend in a straight line.Further, unlike the use of a dedicated fibre where the correlationbetween a position along the fibre length and a corresponding locationwithin the geographical area can be known during installation, selectingan existing optical fibre cable presents some uncertainty to thiscorrelation. For example, optical fibres in telecom network in anexchange may be wound in spools to provide extra length for flexibilityfor repair or splicing purposes. Further, optical fibre lengths canchange upon repair or splicing. It is therefore necessary to calibratethis correlation to accurately map any detected fluctuations in thepositions along the fibre to the corresponding locations of any acousticevents.

In one configuration, the step 204 includes spatially calibratingbetween a position along the optical fibre and a location in thegeographical area. The spatial calibration may include generating anacoustic calibration signal (e.g. a single-frequency tone at 420 Hz +/−5Hz selected to be distinct to typical noise sources in urban centres) atspecific locations of the geographical area to cause fluctuations fordetection along the length of the optical fibre. By restricting theacoustic calibration signal frequency to 420 Hz +/−5 Hz, other acousticnoise sources in the urban centre can be removed. With the removal ofother acoustic noise sources, one strong signal that corresponds to thesingle-frequency tone can be detected, as can be observed in FIG. 1b ataround Channel 1990. As an illustrative example, an operator may travelto specific locations, e.g. to cable pits along the selected opticalfibre cable installation where cable is typically coiled in a lidded pitfor allowing additional cable runs to be made from that point if andwhen necessary. Up to 50 m or more of cable may be coiled in each pit,which are spaced at intervals of x-y m along the cable trench. Theresult is that there is a significant (up to 15% or more) discrepancybetween the cable path length and the geographic path length.

In the geospatial calibration process the calibration acoustic signal isgenerated at each successive pit location and GPS coordinates oflatitude and longitude in decimal degrees are taken at successive cablepits. Alternatively other locations along the cable path may be used,where the calibration acoustic signal is easily detectable, provided theGPS coordinates are recorded or noted. An optical fluctuationcorresponding to the calibration acoustic signal is expected to bedetected at a specific position along the optical fibre. Thecorresponding pair of coordinates corresponding to a location within ageographical region and the position along the optical fibre where thefluctuation is detected forms a geospatial calibration reference point,which then forms part of a look-up table of the type indicated below,which includes further spatial calibration points along the fibre andwithin the geographical area.

TABLE 1 GEOSPATIAL CALIBRATION LOOK-UP TABLE Date Energy Optical OpticalCreated & Latitude Longitude peak Distance Distance Time Name (deg_dec)(deg_dec) Point (m) Start (m) End (m) Width (m) Type 20170714 9883P−33.92095274 151.18087789 9,883.0 9,875.0 9,893.0 18.0 Pit 1147 201707149775C −33.92122648 151.18191622 9,775.0 0.0 Cable 1150 20170714 9193C−33.92222025 151.18731547 9,193.0 0.0 Cable 1204 20170714 5128P−33.92240548 151.18880188 5,124.0 5,118.0 5,129.0 11.0 Pit 1212 20170714 374P −33.92241321 151.18884028 374.0 372.0 376.0 4.0 Pit 1216 201707144235P −33.91819560 151.18831050 4,235.0 4,223.0 4,247.0 24.0 Pit 123720170714 3764P −33.91768567 151.18876948 3,764.0 3,757.0 3,773.0 16.0Pit 1244 20170714 8374P −33.91767414 151.18878031 8,374.0 8,372.08,378.0 6.0 Pit 1247 20170714 8474P −33.91817861 151.18832572 8,474.08,472.0 8,478.0 6.0 Pit 1255

In the table above each point is identified with a date and time stampand an name or reference which is similarly associated with the GPSco-ordinates captured by the data logger on an appropriate GPS enableddevice such as a smart phone. The date logger transmits this informationto the data centre where it is mapped. The peak point of the fluctuationon the optical fibre is logged, together with the end and start opticaldistances in the case of a pit being logged where a fibre loop is likelyto be present. The width reading represents the length of that loop. Thetype column indicates whether the reading is taken at a cable (in whichcase the energy peak point is relevant) or a pit (in which case theoptical start and end distances and width are relevant.

If an acoustic event is detected at a position along the fibre betweentwo calibration points, an interpolation (e.g. linear or nonlinear) maybe used to estimate the location of the corresponding occurrence withinthe geographical area. If an acoustic event is detected at a positionalong the fibre beyond the first and the last calibration points, anextrapolation (e.g. linear or nonlinear) may be used to estimate thelocation of the corresponding occurrence within the geographical area.By using the above geospatial calibration method, substantial variationswhich are encountered in the case of fibre optic networks which are notdedicated to performing location sensing and detecting functions but arerather dedicated communications and enterprise networks of the typecontemplated in the present disclosure can be reduced and accuracy oflocation detection can be improved across the entire network quitesubstantially. This calibration of the optical path length to geospatialposition enables accurate queueing of personnel to threat or cable breakevents in a way that the applicant understands is not possible withexisting methods of determining the location of cable events.

Physical Calibration

The step 204 may further include physical calibration of the opticalfibre selected for acoustic distributed sensing. Unlike dedicated fibredeployment, the disclosed method 200 involves use of an optical fibre ofgenerally uncharacterised properties. For example, its core properties,attenuation properties, and trench properties are generally unknown. Theobtained properties may be used for calibration of the detectedfluctuations.

In one configuration, the step 204 includes obtaining core properties ofthe optical fibre. The core properties may include core diameter and/ornumerical aperture. The core properties may affect the launch power ofthe light source, which in turn affects the intensity of the reflectedlight. For instance, the minimum reflected intensity, which can beincreased by the launch power, is limited by the noise floor of thephotodetector. Based on the core properties, the launch power of thelight source may be adjusted accordingly in step 206 to achieve adesired reach. Alternatively or additionally, the step 204 includesobtaining attenuation properties of the optical fibre. The attenuationproperties may include propagation loss per unit length, existing faultsand/or spliced points. The attenuation properties may affect the reachof the distributed acoustic sensing. For instance, a higher propagationloss lowers the reach. Further, existing faults and/or spliced pointsmay cause a different fluctuation (amplified or reduced) in thereflected light compared to fluctuations in an otherwise fault-free orsplice-free fibre.

Based on the attenuation properties, the launch power may be adjustedaccordingly in step 206 to achieve a desired reach. Still alternativelyor additionally, the step 204 includes obtaining trench properties ofthe optical fibre. Trench properties include properties affected byburial conditions and/or cable enclosure conditions. For instance, anoptical fibre may be enclosed in 100 mm PVC conduit, and/or buried in acement trench, earthenware and an underground tunnel. The trenchproperties may mask or otherwise affect the acoustic signature of anacoustic event. FIG. 4 illustrates an aerial map in the Circular Quayarea in Sydney, Australia. The aerial map is overlaid with multiplesections of optical fibre (represented by at least labels “ch1346” and“ch1384”), each corresponding to a location (corner of Bent and BlighStreets, and corner of Macquarie and Bridge Streets, respectively) nearthe Circular Quay area. Here, different sections of the optical fibresare subject to different trench conditions, which may be obtained fromthe cable supplier or from acoustic measurements by testing site. Thetrench properties, for example obtained via acoustic measurements, maybe used for calibration of the detected fluctuations in step 210, suchas in conjunction with some techniques in alert classification describedin “Fiber Sensing: Optical fiber monitors the arterial networks ofcommerce” referenced above.

Stationary, Slowly Moving or Fast Moving Occurrences

As mentioned above, acoustic events-including objects- being determinedmay be indicative of specific stationary or moving occurrences. Forexample, as illustrated in FIG. 1b , features such as straight lineswith relatively constant gradients are associated with the movingobjects (with the gradients being indicative of speed) that cause therelevant acoustic events detected by the DAS unit 100. The disclosedmethod 200 may include the step of determining whether an acoustic eventis stationary or moving. This determination may include whether a movingacoustic event relates to a slow moving noise source (e.g. drilling,excavating, bore tunnelling, etc.) or a fast moving noise source (e.g.cars, trains, etc). For example, the determination may include comparingan estimated speed (e.g. based on the gradient of a straight line) ofthe acoustic event with a threshold speed value. Where the estimatedspeed of the acoustic event is below the threshold speed value, theacoustic event is determined to be slowly moving or stationary,otherwise it is determined to be fast moving. The disclosed method 200may further include the step of, based on the determination, suppressingthe generation of the alert signal representative of the acoustic event.This suppression is useful in avoiding false alarms, particularly in anurban environment where the number of occurrences of benign urbanactivities (e.g. pedestrian walking, and moving busses and trains) canbe much higher than that of genuine threats (e.g. technical persons orengineers working on optical fibres, drilling, digging and excavation).

For example, in the context of telecommunication infrastructure,detection of a stationary or slowly moving noise source indicates ahigher likelihood of that noise source being a threat, whereas detectionof a fast moving noise source indicates a lower likelihood of that noisesource being a threat. The threat likelihood may be graded (e.g. in ascale of low, medium and high, or in a scale of 1 to 10). The thresholdgrade beyond which a threat alert is (or is not) generated may beadjustable, e.g. dynamically, based on the use-case context. Withoutsuch suppression, a high number of false alarms may be generated to apoint which renders the disclosed method ineffective. In onearrangement, the threshold speed value and/or the threshold grade may beadjusted to reduce the number of false alarms.

Switching

To increase the total length of existing optical fibre cable that can bemonitored from one DAS unit 100, in one configuration, the disclosedmethod further includes switching the transmission of the outgoing lightand the reception of the reflected light to another unused channel orunlit optical fibre for communication, the other optical fibre being inanother bundle of optical fibres in another selected optical fibre cableinstallation having another path extending across another selectedgeographical area.

FIG. 5 schematically illustrates an example of the DAS unit 100optically coupled, e.g. via an optical fibre, to an optical switch 500to form a system 400 of distribution acoustic sensing. The opticalswitch 500 may be located in a data centre or hub, which connects tomultiple optical fibre installations (502 a, 502 b, 502 c, 502 d and 502e). The DAS unit 100 may be collocated with or separately located fromthe optical switch 500. In the case where the DAS unit 100 is collocatedwith the optical switch 500 in a data centre or hub, the DAS unit 100 iswithin close proximity to an ecosystem of optical communication networks(e.g. enterprise networks, cloud provider networks, IP transit providernetworks, internet service provider networks, and telecom carriernetworks, including regional, metropolitan and long-haul networks).Accordingly, the DAS unit 100 at the data centre or hub may beconfigured to selectively access one or more different communicationsnetworks types for distributed acoustic sensing. As mentioned, in oneexample, the selection of an enterprise network provides an advantage ofa relatively large optical fibre cross section.

The optical switch 500 is configured to couple light between the DASunit 100 and any one of multiple optical fibre installations (502 a, 502b, 502 c, 502 d and 502 e). The multiple optical fibre installations 502together span a larger geographical area than would otherwise be spannedby any one of the fibre installations 502 alone. In one configuration,the optical switch 500 time-multiplexes the multiple optical fibreinstallations. For example, transmission of outgoing light and receptionof reflected light is cycled through the multiple optical fibreinstallations and switched to a next installation at regular intervals.

In some circumstances, different installations may detect the sameacoustic event to increase the spatial accuracy of locating the acousticevent. For example, an acoustic event may be detected by bothinstallations 502 b and 502 c but not by installations 502 a, 502 d and502 e. Such detection indicates that the corresponding occurrence islocated in a geographical region between 502 b and 502 c. If onlyinstallation 502 b was interrogated, the detection might present anuncertainty as to where (e.g. either between installations 502 a and 502b or between installations 502 b and 502 c) the occurrence is located.If the acoustic event generates seismic waves, the waves may propagateacross to multiple cables. In this case, the epicentre may betriangulated based a measurement of direction of propagation and time offlight calculations. Now that arrangements of the present disclosure aredescribed, it should be apparent to the skilled person in the art thatthe described arrangements have the following advantages:

-   -   The expense of deploying a dedicated optical fibre for        distributed acoustic sensing is avoided.    -   The ability to measure an asset or acoustic event at a        particular location where it would not otherwise be possible to        locate a dedicated sensor system as the real-estate or land is        owned by another party.    -   The calibration steps adapt an existing installation to        imperfect or non-ideal characteristics that would otherwise be        absent in dedicated optical fibre deployment.    -   Where switching is used, the system can readily be scaled to        expand the geographical area of interest or the total length of        cable to be monitored. Further, switching can increase the        spatial accuracy of locating an occurrence outside the cable.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.For example, any one or more the calibration steps can be usedseparately or in conjunction. All of these different combinationsconstitute various alternatives of the present disclosure.

1. A method of distributed acoustic sensing, the method including:selecting an optical fibre cable installation having a path extendingacross a selected geographical area, the optical fibre cableinstallation including a bundle of optical fibres that form part of anestablished and dedicated fibre-optic communications network, the bundleof optical fibres including an unused channel or unlit optical fibre forcommunication; determining characteristics associated with the opticalfibre and/or the selected optical fibre installation, includinggeospatially calibrating the optical fibre for mapping between one ormore positions along the length of the optical fibre and a correspondingone or more locations in the geographical area; transmitting outgoinglight in the optical fibre; receiving reflected light back scatteredalong the optical fibre, the reflected light including fluctuations overtime; and based on the fluctuations and the determined characteristics,generating an alert signal representative of an acoustic event.
 2. Themethod of claim 1 wherein the fibre-optic communications network is anestablished urban or metropolitan area network.
 3. The method of claim 1wherein the fibre-optic communications network is an enterprise network.4. The method of 3 wherein the enterprise network includes a pluralityof data centres interconnected by optical fibre installations.
 5. Themethod of claim 1 wherein the geospatially calibrating includesgenerating an acoustic calibration signal at or adjacent the one or morepositions along the optical fibre, determining and logging thegeospatial locations of the positions in terms of geographic coordinateson the earth's surface, detecting corresponding fluctuations at the oneor more locations in the geographical area in received reflected lightbackscattered along the optical fibre, and determining the path lengthof the optical fibre corresponding to the one or more geospatiallocations.
 6. The method of claim 5 where the geospatially calibratingcomprises correlating the path length of the optical fibre with thegeographic co-ordinates of the one or more locations to generate a lookup table correlating optical path length with geographic co-ordinates.7. The method of claim 1 wherein generating the alert signal includesdetermining a location of an occurrence of an incident in thegeographical area based on a corresponding fluctuation detected based onthe mapping.
 8. The method of claim 1 wherein the determining thecharacteristics includes acoustically calibrating the optical fibre toreduce impact of unwanted acoustic interference.
 9. The method of claim8 wherein the acoustically calibrating includes applying a spectralfilter to the fluctuations to band-pass or band-reject the unwantedacoustic interference.
 10. The method of claim 9 wherein the selectedgeographical area includes multiple zones corresponding to multiplesections of the optical fibre, and wherein the spectral filter applyingstep includes applying the spectral filter with band-pass or band-rejectcharacteristics based on one of the multiple zones or correspondingsections.
 11. The method of claim 9 wherein the spectral filter applyingstep includes applying the spectral filter with band-pass or band-rejectcharacteristics based on at least one of the time of a day and the dayof a week.
 12. The method of claim 1 wherein determining thecharacteristics includes physically calibrating the optical fibre. 13.The method of claim 1 wherein the physically calibrating includesobtaining any one or more of the following properties of the opticalfibre: core properties, attenuation properties, and trench properties.14. The method of claim 13 wherein the core properties include at leastone of a core diameter and a numerical aperture.
 15. The method of claim13 wherein the attenuation properties include at least one of apropagation loss, existing faults and spliced points.
 16. The method ofclaim 13 wherein the trench properties include at least one of burialconditions and cable enclosure conditions.
 17. The method of claim 1wherein generating the alert signal includes classifying the alertsignal into one or more classes of alerts based on acoustic signaturesof the fluctuations.
 18. The method of claim 17 wherein the selectedgeographical area includes multiple zones corresponding to multiplesections of the optical fibre, each zone or corresponding section beingassociated with generation of one or more selected classes of alerts.19. The method of claim 17 wherein the selected geographical areaincludes multiple zones corresponding to multiple sections of theoptical fibre, each zone or corresponding section being associated withnon-generation of one or more excluded classes of alerts.
 20. The methodof claim 18 wherein the one or more selected or excluded classes ofalerts corresponding to each zone or section of the optical fibre arerelated to at least one of rail monitoring, road monitoring, andperimeter intrusion detection. 21-30. (canceled)